1. Field of the Invention
This invention relates to zoom lenses, and more particularly to zoom lenses of extended range in compact form, while still preserving good stability of aberration correction throughout the extended zooming range.
2. Description of the Prior Art
Though not very compact, many zoom lenses which enable a great increase in the range of variation of the focal length have been proposed. Zoom lenses of this type generally have four lens units, of which the first and third counting from front are axially movable for zooming.
In the zoom lens proposed in U.S. patent application Ser. No. 628,491 filed July 6, 1984 (corresponding to Japanese Patent Application No. SHO 58-128974), the first, second, third and fourth lens units have respectively positive, negative, positive and positive refractive powers from the front, and one zooms by moving the first and third lens units axially in differential relation to each other, while holding the second and fourth lens units stationary. Despite its remaining stationary during zooming, the second lens unit can be made to contribute to the magnification power varying effect by making first lens unit move axially forward as zooming proceeds from the wide angel to the telephoto end. Further, the third lens unit not only compensates for the image shift during zooming, but also contributes to a magnification power varying effect. In such a way, the prior art has achieved an increase in the zoom ratio of the zoom lens.
This type of zoom lens becomes conveniently compact for portability when set in the wide angle position, because its first lens unit is retracted toward the image plane. Also, the second lens unit whose refractive power is relatively strong is fixed in position. Thus, there is no possibility of introducing eccentricity of axis with zooming. Hence, the loss of the imaging performance due to the assembling operation can be minimized.
Furthermore, maintenance of the second lens unit in the accurately adjusted position within the lens barrel, allows loosening of the tolerance to which the other lens units must be held.
Also, the possibility of adjusting the position of the second lens unit with great accuracy allows for a further increase of its refractive power. As a whole, it is possible to achieve a shortening of the entire lens system in the longitudinal direction.
However, the use of this type of construction tends to increase the range of variation in image aberrations, particularly astigmatism in the intermediate zone of the picture frame. This tendency becomes prominent as the zoom range increases to, for example, 4 which the present invention attempts to obtain. In order to make the zoom lens compact, the refractive powers of the first, second and third lens units are necessarily increased. This also involves a large increase in the range of astigmatism variations. In the intermediate focal length positions, particularly the astigmatism and the spherical aberration vary in opposite directions so that their image surfaces no longer coincide.
In the above-described zoom lens, where the third lens unit is regarded as the compensator, the differential speed of movement of the third lens unit to that of movement of the first lens unit, or their ratio A (hereinafter called "cam inclination"), may be expressed by the following formula: EQU A=-.beta..sub.II.sup.2 .multidot..beta..sub.III.sup.2 /(1-.beta..sub.III.sup.2)
where .beta..sub.II and .beta..sub.III are the magnification powers of the second and third lens units respectively. To use the third lens unit as the compensator, the conditions: .beta..sub.III .noteq..+-.1 must be satisfied through the zooming range. Because the third lens unit's contribution to the increase in the zoom ratio increases, if the magnification power .beta..sub.III is made to operate on the enlarging side of -1, the angle of cam inclination in either of the wide angle and telephoto ends increases greatly. Therefore, a large driving power is required for moving the third lens unit to or away from either of the wide angle and telephoto ends. This imparts a large stress to the operating mechanism for the zoom lens.
Such tendencies are intensified as the zoom ratio increases and the bulk size decreases.